1. Field of the Invention
The present invention relates to a speech processing device for radioelectric equipment adapted for transmission and reception of voice signals, usable more especially in radiotelephony.
2. Description of the Prior Art
Analog speech processing systems are known which transmit by radio, by means of variable attenuation at transmission, a practically constant level of the voice signal such that the level can be maintained for a time close to a few tens of milliseconds. One such known system is LINCOMPEX which substantially improves the signal/noise ratio obtained at reception. In these systems, the value of the attenuation introduced at transmission is transmitted through a frequency multiplexed auxiliary channel. By frequency modulation of a sub-carrier situated at the top of the telephone band, so as to control an expander situated at the level of the receiver, where the expander is an attenuator operating reversely to the transmitter attenuator, the speech level existing before compression of the voice signal may be found. These purely analog systems are used in radiotelephony in the decametric wave range. An improvement to the LINCOMPEX system is known under the name SYNCOMPEX. In the SYNCOMPEX system the voice signal is processed and transmitted by packets of finite duration (13.33 milliseconds) over a main channel and over an auxiliary frequency radioelectric channel which transmits attenuation values in digital form. The corresponding codes are transmitted by a frequency jump modulation system of the "FSK" type, i.e. frequency shift keying, which operates with diversity coupling on two sub-carriers introduced into the speech band.
Although providing good performance, these systems do not suppress the echo phenomena generated either by cross-talk between the transmission and reception channels or else by microphones placed close to reception loud-speakers.